With developments in electronics technology, particularly information-related technology typified by computers, demand for information-recording media such as magnetic disks, optical disks, and magnetooptical disks has increased quickly. The main component elements of magnetic storage devices such as computers are a magnetic recording medium and a magnetic head for magnetic recording and reproduction. Flexible disks and hard disks are known as magnetic recording media. Among these, there exist substrate materials in the form of aluminum substrates, glass substrates, ceramic substrates, carbon substrates, and the like for hard disks (magnetic disks). In practical terms, aluminum substrates and glass substrates are primarily employed, depending on size and application. However, with the reduction in size of the hard disk drives employed in notebook computers and the increased density of magnetic recording, the requirements imposed on disk substrate surface flatness and thickness reduction have become quite stringent. Thus, aluminum substrates, which afford poor processability, strength, and rigidity, are inadequate. Accordingly, glass substrates for magnetic disks affording high strength, high rigidity, high impact resistance, and high surface flatness have made an appearance.
In recent years, vertical magnetic recording methods have been employed in an attempt to achieve higher recording densities in information-recording media (for example, high recording densities of 100 Gbit/inch2 or greater). The use of vertical magnetic recording methods permits a marked increase in recording density. Additionally, achieving a high recording density requires greatly reducing the distance (referred to as the “flying height” in magnetic recording media) between the heads for reading and writing data (such as magnetic heads) and the medium surface, to 8 nm or less. However, when the substrate surface is not smooth, irregularities on the substrate surface are reflected on the medium surface, precluding a reduction in the distance between the heads and the recording medium, and hindering improvement in linear recording density. Thus, achieving high recording density through the use of a vertical magnetic recording method requires a glass substrate for use in an information-recording medium with a markedly better degree of flatness than in the past.
Since adhesion of foreign matter to the glass substrate of an information-recording medium is unacceptable, adequate cleaning must be conducted. Cleaning agents such as acids and alkalis are employed in cleaning. However, when the chemical durability (acid and alkali resistance) of the glass constituting the substrate is inadequate, the manufacturing process ends up producing surface roughness, even when the substrate surface is finished for flatness. Even slight surface roughness makes it difficult to achieve a medium substrate with the level of flatness required by vertical recording methods. Thus, increasing the linear density of an information-recording medium requires a substrate material having good chemical durability.    [Patent Document 1] International Patent Application Publication No. 2007-142324 (WO 2007/142324) (the entire contents of which are hereby incorporated by reference).